WASHINGTON -- Spies hacked into U.S. military computers, copying data that could threaten the effectiveness of U.S. fighter jets, The Wall Street Journal reported Tuesday.

In recent months, the newspaper said, someone was able to get into

Pentagon's $300 billion Joint Strike Fighter project and take several terabytes of information concerning design and electronics systems. Officials told the Journal it could make it easier for America's enemies to defend against the F-35 Lightning I fighter jet.

The Journal reported the Air Force's air-traffic-control system also was breached in recent months.

"There's never been anything like it," one source told the Journal, adding that other military and civilian agencies as well as private companies are affected. "It's everything that keeps this country going."

While former U.S. officials told the newspaper it appeared the attacks originated in China, the specific identity of the attackers and the extent of the damage to the U.S. defense program wasn't known. However, the hackers weren't able to access the most sensitive material, which is stored on computers not connected to the Internet, the Journal said.